Two of the regular characters - Jack Harkness, and Toshiko Sato - originally appeared on the parent series,
"Doctor Who" (2005). Toshiko (
Naoko Mori) was the government forensic pathologist called in to perform the autopsy on the alien corpse in
"Doctor Who: Aliens of London (#1.4)" (2005). Jack (
John Barrowman) first appeared in
"Doctor Who: The Empty Child (#1.9)" (2005) and became one of the Doctor's companions. He was last seen in
"Doctor Who: Journey's End (#4.13)" (2008).
Eve Myles played a similarly named character on the parent series in the episode,
"Doctor Who: The Unquiet Dead (#1.3)" (2005). It was heavily suggested that the two are related in
"Doctor Who: The Stolen Earth (#4.12)" (2008) when the Doctor and Rose Tyler mention that she resembles the earlier character and she confirms that her family has lived in Cardiff since the 1800s.
Freema Agyeman was a series regular in series three of
"Doctor Who" (2005), as the Doctor's companion 'Martha Jones'. Agyeman later appeared in
"Torchwood: Reset (#2.6)" (2008) and the two following episodes as the same character.
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